Secreted Protein, Acidic, Rich in Cysteines (SPARC), also known as osteonectin, is a 303 amino acid glycoprotein which is expressed in the human body. SPARC expression is developmentally regulated, with SPARC being predominantly expressed in tissues undergoing remodeling during normal development or in response to injury. See, e.g., Lane et al., FASEB J., 8, 163-173 (1994). For example, high levels of SPARC protein are expressed in developing bones and teeth, principally osteoblasts, odontoblasts, perichondrial fibroblasts, and differentiating chondrocytes in murine, bovine, and human embryos. SPARC also plays important roles in cell-matrix interactions during tissue remodeling, wound repair, morphogenesis, cellular differentiation, cell migration, and angiogenesis, including where these processes are associated with disease states. For example, SPARC is expressed in renal interstitial fibrosis, and plays a role in the host response to pulmonary insults, such as bleomycin-induced pulmonary fibrosis.
SPARC is differentially expressed in tumors and its surrounding stroma in various cancers in comparison to the normal tissue, with the pattern depending on the type of cancer. Thus, there is no unifying model which explains all facets of its function and contribution to the development and progression of cancer. In one pattern, increased SPARC expression have been reported in breast cancer (Bellahcene and Castronovo, 1995; Jones et al., 2004; Lien et al., 2007; Porter et al., 1995), melanoma (Ledda et al., 1997a), and glioblastomas (Rempel et al., 1998). Increased SPARC expression plays a role in tumor promotion or progression in these cancers.
Accordingly, SPARC over expression in inflammation and some cancers makes a SPARC potential target for diagnosis and therapy.